[4.x] Add field_caption for Documents

(fixed manually at prezima ;))

Unable to add field_caption to documents because:

While there is no button to add existing fields, for buggy core reasons I guess..

Ah.. fixed by field tools..

Preset config done.

@thomas.frobieter: Config was now added in the parent issue. Can this be set FIXED or are styling tasks remaining for you?

I need to check the output and probably fix the styling, so yes, leave it open for me.

@Grevil: We'll also need an update hook here to create the field when migrating from 3.x:
In an update hook, check if the field is already present in documents (that's the case in our preset). If yes, skip the import, otherwise import the new field and set the weight in form and display accordingly.

Note: We created the field in our preset manually already, so that we're not blocked.